The present invention relates to a method and apparatus for indicating the repetition intervals of a specified component of a composite, cyclically-repeating electrical signal. The invention is particularly applicable for displaying the fetal R-wave with respect to the maternal R-wave, and therefore is described below with respect to this application, although it will be appreciated that the invention can be used in other applications as well.
It has been generally recognized that whereas the fetal ECG (electrocardiographic) signals can sometimes be picked up at the mother's abdominal surface even earlier than about 20 weeks of gestation, measurement and analysis of the fetal component in the abdominal recording is a complex problem. There are several reasons for this:
(1) Electrical signals from the fetal heart, when measured at the mother's abdominal surface, are very low in amplitude, often less than 10 .mu.V. They may be weaker than myoelectric signals produced by movement of the mother, particularly during labor.
(2) The strongest signal picked up at the maternal abdominal surface is the maternal electrocardiogram, which may be anywhere from 5 to 1000 times higher in amplitude than the fetal signal. This signal must be removed or otherwise taken into account if the abdominal ECG is to be used for reliable assessment of the fetal ECG.
(3) There is no commonly agreed answer to the question whether the medium of electrical conduction (of fetal signals to maternal abdominal electrodes) should be considered even approximately homogeneous, and there are indications that this pattern of conduction may vary over gestation. This complicates the analysis of the fetal electrocardiogram even when it can be measured. Further the appearance of the vernix covering the fetal skin during the last three months of fetal life, introduces a high electrical impedance between the source of fetal electrocardiographic signals and the measuring electrodes. This can substantially reduce the amplitude of the fetal electrocardiogram as it is measured on the maternal abdominal surfaces. This is probably one of the reasons why the quality of abdominally obtained fetal ECG's deteriorates in the period between about 26 and 33 weeks of gestation.
(4) Fetal position in-utero varies with time and varies among patients. The location of the site of placentation (which may be at least partly responsible for the electrical conduction of fetal heart signals to the mother's abdominal surface) also varies among patients. Therefore, even if the fetal electrocardiogram can be obtained from the maternal abdomen, standardizing it is a much more complex matter than in case of adult electrocardiograms.
Reliable, non-invasive observation of the fetal ECG in abdominal recordings is of significant diagnostic importance in the monitoring of pregnancies, not only for fetal heart rate assessment, but also for observation of congenital cardiac malformations and transient difficulties before and during labor. Furthermore, a reliable abdominally obtained fetal electrocardiogram may be used to assess fetal heart rate variability in the antepartum period of gestation when it is not possible to obtain a fetal scalp lead electrocardiogram.
One approach for indicating the repetition intervals of the fetal R-waves is merely to remove the maternal R-waves from the abdominal ECG signal (which includes both the maternal R-waves and the fetal R-waves), thereby leaving only the fetal R-waves. However, such a system is very sensitive to baseline changes originated by body movements; moreover, it cannot detect a fetal R-wave falling on a maternal R-wave. A number of other techniques have been proposed for indicating the repetition intervals of the fetal R-waves, but none has been proved to be entirely satisfactory because of the difficulties discussed above.
An object of the present invention is to provide a novel method, and also novel apparatus, for indicating the repetition intervals of a specified component of a composite, cyclically-repeating electrical signal, which method and apparatus are particularly useful for indicating the fetal R-waves.